1. Field of the Invention
The present invention relates to a method for recording a signal by applying a biasing magnetic field to a magneto-optical recording medium while a light beam (such as a laser beam) is irradiated to the medium.
2. Related Background Art
An optical memory which records or reproduces using a light beam has recently been developed for use as a high density and large capacity memory. Magneto-optical recording medium has also been attracting interest for use as a rewritable optical memory. In such a magneto-optical medium, a signal is recorded by orienting directions of magnetization of a magnetic layer of the medium along a predetermined direction, and irradiating a light beam which is intensity modulated in accordance with a record signal while a biasing magnetic field is applied to the medium in the opposite direction to the predetermined direction. The signal is reproduced by irradiating a polarized beam to the medium and observing the change of direction of polarization by a magneto-optical effect.
However, in the above recording method, it is necessary to orient the directions of magnetization of the magnetic layer, that is, erase the previously recorded signals before a signal can be rewritten. On the other hand, in Japanese Laid-Open patent application No. 51-107121, a biasing magnetic field whose polarity is reversed in accordance with a signal is applied while a light beam of a constant intensity is irradiated to the medium so that the overwriting is attained without the erase step. EPO Laid-Open patent No. 258,978 (corresponding to .fwdarw. U.S. patent application Ser. No. 475,941) and West Germany Laid-Open Pat. No. 3,619,618 disclose overwriting by irradiating an intensity-modulated light beam to a magneto-optical recording medium having two exchange-coupled magnetic layers.
On information other hand, recording density and the speed are limited if information (representing each record bit) is a binary signal representing an upward or downward direction of magnetization of the magneto-optical recording film, because the recording density of the currently available magneto-optical recording medium is essentially determined by a wavelength of a semiconductor laser used for recording the signal (approximately 800 nm). For example, to double the recording density, requires a semiconductor laser having a wavelength of approximately 800/.sqroot.=570 nm which is not currently available is distant.
However, if the record bit recorded on the magnetic layer can have one of tetra values, the recording density is doubled. If the tetra-value signal is recorded and reproduced at the same speed as that for a conventional binary signal, the recording/reproducing speed is also doubled. A tetra-value recording method is disclosed in U.S. Pat. No. 4,612,587 wherein a signal is recorded by irradiating two light beams having different wavelengths and independently intensity-modulated are irradiated to a magneto-optical recording medium having two magnetic layers having different absorbing wavelengths.
However, U.S. Pat. No. 4,612,587, the directions of magnetization of the two magnetic layers must be oriented before the signal is recorded, and the overwriting is not attained. Further, the two light beams which are used require two light sources so that the recording apparatus is complex in construction and expensive.